Many have branded the case a miscarriage of justice, pointing out that she was tried as an adult and sent to prison for prostitution and murder.

A petition on MoveOn.org calling for Brown to receive a presidential pardon has so far garnered over 120,000 signatures across the world.

"Misguided and lost children is an epidemic that we as adults should be responsible for; not the child. Although murder was committed in this case; adults have committed murder and received a lesser than a life sentence," it reads.

The Cyntoia Brown case

In 2004, after years of working as a prostitute, taking drugs and being abused, 16-year-old Cyntoia Brown was hired by Nashville real estate agent Johnny Allen. Hours after their meeting, she fatally shot him.

During the trial, Brown claimed that she pulled the trigger because he had boasted about his military background and showed off his guns before they had sex.

A photo of Cyntoia Brown at Tennessee Prison for WomenTennessee Prison for Women

She testified, "He was a sharp shooter in the Army. I'm sitting here thinking if he does something, what am I going to do?"

When he turned over suddenly as they laid in bed, she said she believed he was reaching for a gun and feared for her life. Taking a 40-calibre handgun from her handbag, she shot him in the back.

"I shot him," she said in a conversation that was recorded shortly after the crime. "I executed him."

Her legal team argued that what transpired that night was the culmination of a traumatic childhood which saw her regularly beaten, choked, dragged and raped for years.

Brown also suffers fetal alcohol syndrome, which experts say is a "severe mental disease and disorder". Her biological mother admitted to abusing alcohol while pregnant with Brown and worked the streets prostituting to support a drug addiction.

Brown was forced into sex trafficking by an abusive drug dealer named Kutthroat at the age of 13, the AP reported.

Despite her age, Brown was tried as an adult. The jury rejected her claim of self-defence and found her guilty of first-degree premeditated murder, first-degree felony murder and especially aggravated robbery.

In 2011 filmmaker Daniel Birman released a documentary about Brown entitled Me Facing Life: Cyntoia's Story, which gave an insight into the physical, the sexual and verbal abuse she suffered as a child before Allen solicited her for sex.

That same year, Tennessee laws were changed to prohibit anyone 18 or younger to be charged with prostitution. "She was picked up by a 43-year-old man. Cyntoia was 16 years old," Birman told Fox 17.

Brown has already served 13 years of her life sentence, and won't be eligible for parole for another 51 years. According to Newsweek, she graduated from Lipscomb University in December 2015 with an associate's degree she earned while in prison and is working towards her bachelor's degree.

"I'm always moving forward. I have the stigma of being an inmate," Brown said of her achievement "But I try not to get caught up in the negative. I have to stay positive to survive."

The system has failed. Itâs heart breaking to see a young girl sex trafficked then when she has the courage to fight back is jailed for life! We have to do better & do whatâs right. Iâve called my attorneys yesterday to see what can be done to fix this. #FreeCyntoiaBrownpic.twitter.com/73y26mLp7u